watchOS 4.0 (build 15R372) is an official Apple over-the-air update for the Apple Watch Series 1 (38mm), released on 2017-09-12T19:05:11Z. The update package is 539.15 MB. This is a delta (partial) update that requires 3.2.3 already installed. This update is no longer signed by Apple, meaning it can no longer be applied on-device through Settings.

Payload file: 74d90d33ec521bf09b55922faa98b83d34607652.zip.